Conquering an Infinite Cave. Hidden in Vietnam's rugged Phong Nha-Ke Bang National Park near the border with Laos, Hang Son Doong Cave is part of a network of 150 or so caves—many still not surveyed—in the Annamite Mountains.  “More men have landed on the moon than have entered this cave,” said Howard Limbert, the British expedition leader, who was among the first to cave in Vietnam since the 1970s. He compared the discovery of this giant chamber to the discovery of a new Mount Everest underground. “It’s really a once in a lifetime adventure to explore the unknown.”